The present invention relates to processes and articles for effecting removal of ink from paper and the like. More particularly, the present invention relates to a substrate having a silicone pressure sensitive adhesive composition on one side and, optionally, a silicone release coating on the opposite side thereof, so that tapes can be prepared therefrom. Type-correcting tapes made according to the present invention can be unwound without the adhesive blocking to the reverse side of the tape. When the tape is then activated by striking with a typewriter key, the adhesive transfers the ink from the surface of the paper or typing substrate to the type-correcting material.
A vertically cohesive ink is a specially formulated ink which is adapted to have greater internal cohesion than external adhesion to the substrate to which it is applied. Inks having this property generally are deposited as a thin, integral, coherent film on typing paper or other suitable cellulosic substrate. This allows the ink character to be contacted on its surface by an adhesive means and plucked from the surface of the paper as an integral unit, that is, without fracturing internally and without leaving ink remaining on the paper. Correctable typewriter ribbons employing such vertically cohesive inks are commercially available.
The ink-removing or type-correcting article must be made from a discriminating adhesive. A discriminating adhesive is one which has a greater affinity for vertically cohesive ink than for paper or the like. The ability of an adhesive to discriminate between the paper and the ink depends on a variety of factors, including the chemical and physical characteristics of the adhesive. In addition to having a greater affinity for vertically cohesive ink than for the paper, it is also necessary that the adhesive have greater internal cohesion than external adhesion so that all of the ink will be transferred from the surface of the paper or cellulosic substrate to the correction tape. U.S. Pat. Nos. 3,731,781 and 4,085,838 disclose compositions formed from various classes of organic resins or polymers which have acceptable discriminating ability. However, such organic-based correction tapes suffer from poor unwinding when in use as a result of excessive blocking by the adhesive.
Therefore, there is a need for an improved correction tape which employs a discriminating adhesive but which also may be easily unwound without blocking. Accordingly, a ink-removing article wherein one side of a backing or substrate is coated with a silicone pressure sensitive adhesive for removing the ink from the paper, and, optionally, the opposite side thereof has a silicone release coating which insures that a tape made therefrom will unwind during use without the adhesive blocking to the substrate would be very desirable. If a thinner backing material and adhesive layer could be employed than is presently required for organic-based type-correcting tapes, an increased amount of correcting tape could be provided within the limited amount of space available on a typewriter, thus reducing the frequency for changing correction tapes.
Both pressure sensitive adhesives and release coatings are known in the art of silicone chemistry. In general, the technology of pressure sensitive adhesives and tapes is described in Goodwin, Jr., U.S. Pat. No. 2,857,356, Dexter, U.S. Pat. No. 2,736,721, and Currie et al., U.S. Pat. No. 2,814,601, all of which are incorporated herein by reference. Such pressure sensitive adhesives preferably comprise a mixture of a siloxane resin, a siloxane gum and an agent for accelerating the curing of the composition by heating. It is also preferable to employ a solvent as a carrier for the adhesive and to heat the composition to drive off the solvent prior to curing the adhesive to a cohesive pressure sensitive layer. In order to prevent adhesion of the pressure sensitive adhesive to its own backing when it is rolled up into tapes, a suitable anti-blocking or release coating is interspersed between the surface of the pressure sensitive adhesive and the adjacent uncoated surface of the backing for the pressure sensitive adhesive. Examples of such release coatings are described in Leavitt, U.S. Pat. No. 2,985,545 and in deMontery and Zurlo, U.S. Pat. No. 2,985,544, both of which are incorporated herein by reference. The basic concept involved in these silicone release coatings is the cross-linking of a silanol-stopped silicone fluid on the surface of a substrate by the use of a cross-linking agent. Such cross-linking is generally initiated by a catalyst such as, for example, platinum, which causes copolymerization of the polysiloxane composition.
Those skilled in the art appreciate that the siloxane gum is the continuous phase of the pressure sensitive adhesive in which phenyl units can be substituted for methyl units to change the properties of the adhesive, as for example, its aggressiveness. A siloxane resin, typically a resin which is comprised of monofunctional and tetra or quadrifunctional units (known in the art as an MQ resin), is reacted with such siloxane gum in formulating both methyl-based and phenyl-modified pressure sensitive adhesives. The properties of pressure sensitive adhesive also depend upon the resin to gum ratio. For example, high resin content adhesives are relatively tack-free at room temperature and become increasingly tacky with the application of heat and pressure whereas high gum content adhesives are extremely tacky and aggressively adhesive at room temperature. Because silicone pressure sensitive adhesives are known for being very aggressive, it would not be expected that a silicone pressure sensitive adhesive could be formulated which has greater internal cohesion than external adhesion to the material to which it is attached and which exhibits the discrimination necessary for effecting removal of vertically cohesive inks from paper and the like.
The prior art discloses that the presence of phenyl groups in the polysiloxane gum imparts to the presssure sensitive adhesive a marked increase in adhesion and tackiness above that realized by employing a polysiloxane gum containing only silicon-bonded methyl groups in the preparation of the adhesive. Inasmuch as the art of pressure sensitive adhesives is generally concerned with improving the aggressivness of such adhesives to any surface rather than to providing compositions which are selectively aggressive, the prior art nowhere discloses the novel ink-removing article and process of the present invention. For example, O'Malley, U.S. Pat. No. 4,039,707, discloses a pressure sensitive adhesive wherein it is essential that the number of silicon-bonded phenyl groups be maintained such that for each 2 to 75 phenyl groups attached directly to silicon by carbon-silicon linkage, there is also present from 98 to 25 silicon-bonded methyl groups. Hahn et al., U.S. Pat. No. 3,983,298, discloses a pressure sensitive adhesive which is not phenyl modified; however, it is disclosed therein that in order to obtain a suitable pressure sensitive adhesive composition the resin must be present in from 50 to 60 parts by weight and the polydiorganosiloxane gum must be present within the range of from 40 to 50 parts by weight, the total parts of resin and gum being 100 parts.
It is also known in the art that phenyl adhesives are not compatible with methyl adhesives and other methyl polymers. Thus, when phenyl adhesives are applied and cured over a cured film of methyl-based silicone paper release coating, the high-phenyl adhesive can be transferred to other surfaces. However, when methyl-based adhesives are applied and cured over a cured film of methyl-based silicone paper release coating, the methyl adhesive would not be expected to be easily transferred to other surfaces without blocking or transferring the adhesive to the tape backing. Thus, it is surprising to find that certain methyl-based adhesives having a resin to gum ratio within a critical range are easily and effectively released from a methyl-based silicone paper release coating.
Accordingly, it is an object of the present invention to provide a type-correcting article comprising a silicone pressure sensitive adhesive applied to a backing material wherein the pressure sensitive adhesive has a greater affinity for a vertically cohesive ink and its own backing material than it does for a cellulosic substrate such as paper or the like.
It is also an object of the present invention to provide an adhesive which has greater internal cohesion than external adhesion so that the article of the present invention will effectively remove substantially all of a vertically cohesive ink from the paper or cellulosic surface to the backing material having the pressure sensitive adhesive thereon.
It is still a further object of the present invention to provide such a type-correcting article in the form of a tape wherein a methyl-based silicone pressure sensitive adhesive is applied to one side of the tape or backing material and wherein a methyl-based silicone release coating is applied to the other side thereof such that the tape or backing material can be unwound without blocking or offsetting of the adhesive to the adjacent backing of the tape.
Other objects and advantages of the present invention will be obvious from the following detailed description.